How to Use 'cat' and 'tac' Commands with Examples in Linux

The cat command is a fundamental Linux utility that reads files sequentially and displays their contents to standard output. The name is derived from its function for concatenating and listing files. The tac command (which is "cat" spelled backwards) performs a similar function but displays file contents in reverse order, printing the last line first.

Basic cat Command Usage

The simplest usage of cat is to display file contents:

$ cat text.txt

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This command reads the file and displays its content to stdout (standard output) on your terminal.

Concatenating Multiple Files

The cat command can display multiple files sequentially:

$ cat text.txt text2.txt text3.txt

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Combining Files with Redirection

You can concatenate multiple files into a single new file using the > redirection operator:

$ cat text.txt text2.txt > text3.txt

This command joins the contents of text.txt and text2.txt and writes them to text3.txt:

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Copying Files

The cat command can copy content from one file to another location:

$ cat text.txt > /tmp/file.txt
$ cd /tmp/
$ cat file.txt

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Creating New Files

You can create a new file using cat with redirection:

$ cat > abc.txt

After running this command, you can type content and press Ctrl+D to save and exit.

Using the tac Command

The tac command is the reverse version of cat, displaying file contents from bottom to top:

$ tac text3.txt

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Practical Use Case − Log File Analysis

The tac command is particularly useful for debugging log files, as it reverses the chronological order to show the most recent entries first:

$ tac /var/log/dpkg.log

2016-12-12 11:48:30 startup packages configure
2016-12-12 11:48:30 status installed sqlitebrowser:amd64 3.9.0ubuntu1
2016-12-12 11:48:30 status half-configured sqlitebrowser:amd64 3.9.0ubuntu1
2016-12-12 11:48:29 status unpacked sqlitebrowser:amd64 3.9.0ubuntu1
2016-12-12 11:48:29 configure sqlitebrowser:amd64 3.9.0ubuntu1
2016-12-12 11:48:29 startup packages configure
...

Key Differences

Conclusion

The cat and tac commands are essential Linux utilities for file manipulation and viewing. While cat displays files in their original order and supports concatenation and file creation, tac provides reverse-order display that's particularly valuable for log file analysis and debugging tasks.